1. Field of the Invention
The present invention relates generally to a method for varying a video transfer rate in a Wireless Broadband (WiBro) network, and in particular, to a method for sensing end-to-end channel status, and a method for varying an encoding bit rate of a transmission side's encoder by integrating channel statuses of transmission/reception sides under a category of the sensed statuses, thereby improving the quality of video data during transmission of the video data.
2. Description of the Related Art
A video streaming service in the wireless network was hard to be commercialized for its high bandwidth and strict restriction on the transmission delay time. Recently, however, due to an increase in wireless access rate and the development of video compression technology, the video streaming service in the wireless network is increasingly available.
WiBro (defined in IEEE 802.16e), which is Korean-style Mobile Worldwide interoperability for Microwave Access (Mobile WiMAX), has attracted attention of mobile carriers as the next generation mobile communication network, providing a bandwidth of several tens of Mbps and a mobility of several tens of Km/H, and started its commercialization on 2007, starting with Best-Effort (BE) service.
Generally, the term ‘WiBro’ refers to a super high-speed Internet service in which the user can access the Internet at a high transfer rate of about 1 Mbps using a portable terminal anytime anyplace while at a standstill and on the move, and can enjoy various information and contents. WiBro, which uses a 2.3-GHz frequency band, guarantees a seamless wireless Internet service within a 1-Km cell radius even while the user moves at a speed of 60 Km/H or higher, and it is a new service by which the user can enjoy the wireless Internet at a lower fee. The currently available WiBro technology can support a mobility of 70 Km/H and a transfer rate of up to 1 Mbps per subscriber, and when commercialized, it is expected to support a mobility of 100 Km/H and a transfer rate of up to 3 Mbps. Therefore, with use of the WiBro Internet, the user can enjoy movies on the street or in the running vehicle, and can enjoy on-line games and web surfing as if he/she enjoys the Internet in the room using the wire network.
The WiBro system, a technology evolved one step further from the 2.3-GHz Wireless Local Loop (WLL) technology, covers a 4th generation mobile communication area, and has a field broader than International Mobile Telecommunication (IMT)-2000 which is 3rd generation mobile communication. Accordingly, WiBro is called a 3.5th generation mobile communication technology.
WiBro has both a characteristic of the existing mobile terminal and a characteristic of a Broadband convergence Network (BcN) that converges communication, broadcasting and Internet. Therefore, it has a type in which on/off-line wire/wireless communication is combined with a broadcast service, and the multimedia service requiring a real-time property, such as Voice over IP (VoIP), Video Telephony (VT), Personal Broadcasting, etc. especially over the WiBro network, is the factor directly linked to the success of WiBro.
During operation of a video transfer application over the WiBro network, such as VT and Personal Broadcasting, WiBro controls a level of Modulation & Coding Selection (MCS) and power of a terminal depending on characteristic parameters of a WiBro wireless channel, such as Carrier to Interference and Noise Ratio (CINR), transmission (Tx) power, etc, and increases efficiency of the entire data throughput by an algorithm of a scheduler based thereon.
However, the available bandwidth continuously varies due to occurrence of traffic congestion and a characteristic of wireless channels that suddenly change because of a property of the WiBro system using the wireless network, and also due to a change in the number of users in the cell. In other words, in WiBro, unlike in the wire network, physical medium characteristics such as data transfer rate can abruptly change according to characteristic and environment of the wireless medium. The change in the characteristics can hardly be estimated. This does not raise any significant problem when the user simply enjoys a service such as Internet search, but may cause significant service degradation in terms of delay and jitter for the multimedia service such as moving picture, causing interrupted reproduction of moving images and degradation of the image quality. In addition, a service requiring guarantee of its bandwidth may suffer significant service degradation in terms of Quality of Service (QoS). Since the users' demand for multimedia also increases along with the development of the network, it is recognized that the QoS guarantee issue should necessarily be considered during development of the communication system.
In order to solve this problem, the existing WiBro terminal, when it attempts to transmit video data to the other party, estimates a bandwidth to be allocated thereto by sensing an end-to-end channel status, and varies its video transfer rate according thereto. That is, for a good channel status, the WiBro terminal increases the video transfer rate by increasing an encoding bit rate of a video encoder, and for a poor wireless channel status, the WiBro terminal decreases the encoding bit rate of the video encoder, solving the problem. The channel status estimation method is generally performed depending on a calculated delay exchanged between terminals through RTP/RTCP (Real Time Transport Protocol).
However, even though the same-size data is transmitted and received in the same WiBro network situation, a significant deviation of the delay may occur due to application of Adaptive Modulation and Coding (AMC) and Repetition. Therefore, there is a demand for an absolute reference by which the delay having a relative value can be compensated for.